Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
One type of memory is a non-volatile memory known as Flash memory. A flash memory is a type of floating-gate memory device that can be erased and reprogrammed. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.
Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a column of memory cells are coupled in parallel with each memory cell coupled to a bit line. In NAND flash architecture, a column of memory cells are coupled in series with only the first memory cell of the column coupled to a bit line.
An advantage of NAND flash architecture is that it facilitates a smaller array size due in part to a smaller word line pitch. However, programming voltages of NAND flash architecture are generally higher than those of NOR flash architecture.
Because the programming voltages of NAND flash architecture are generally several times the supply potential, it becomes difficult to pass these high-voltage control signals through the circuit without loss. In general, an n-channel field-effect transistor (nFET) with a positive Vt requires a gate voltage higher than the voltage being applied to its drain in order to pass the drain voltage to the source.
Voltages higher than the supply potential are typically generated internally to a memory device using a charge pump or other voltage generator. Because a voltage generator is required for each voltage concurrently utilized by the device, it is preferable to minimize the number of voltage levels required by the device.
Memory devices generally include some type of boost circuit to provide sufficient gate voltage to an nFET passing the high programming voltage. While the gate voltage must generally be above the programming voltage by a value equal to the threshold voltage, Vt, of the nFET, higher gate voltages will permit correct operating voltages and improved programming speed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate methods and circuits for passing high-voltage control signals in an integrated circuit device.